1. Field of the Invention
The present invention relates to a compressor control device and to a method for controlling a compressor.
2. Discussion of the Related Art
Household and small size industrial appliances, such as refrigerators, freezers or air conditioning systems, include systems that are provided with a compressor driven by an electric motor. A control device, normally based on a switching circuit, controls operation of the motor, and selectively turns it on and off, according to certain requirements.
Under some circumstances, it may happen that the compressor is stalled when the control circuit intervenes and attempts to start the motor. In this case, the control circuit is subject to risk of damage, due to overcurrents that may occur.
Several solutions have been proposed so far, in order to reduce risks of damage and high power consumption associated with stall conditions.
According to a first known solution, the condition of a stalled compressor is detected by means of a thermo-mechanical switch, that breaks the current supply within a given time, if the current remains high. Response of thermo-mechanical switches, however, is not sufficiently fast and protection may fail. Moreover, even in case of timely response, solutions based on thermo-mechanical switches suffer from considerable power consumption, because a constant current, that is several times greater than nominal operative currents, continues flowing until switches are opened.
Another known solution consists of coupling a resistor in series with the control device, in order to sense the current supplied to the electric motor of the compressor. Safety measures are activated when sensed current is higher than a predetermined current threshold. Use of a series resistor affords timely reaction to compressor stall conditions, but also entails higher manufacturing costs, because the resistor has to be large both as to power requirements and to size. In addition, when the compressor is not stalled, the large series resistor seriously impairs power consumption.
Also other control circuits have been proposed, which are configured to detect phase shift between windings of the compressor motor. However, these control circuits need to include special processing units and dedicated components to sense and compare phases, which results in increased cost and size of the devices.